Exemplary embodiments of the present invention relate to particulate filters for diesel exhaust systems. More particularly, exemplary embodiments of the present invention relate to particulate filter assemblies that utilize pleated ceramic filter media and can be incorporated into diesel exhaust treatment devices.
Because regulatory agencies have recently mandated the reduction of particulate emissions in diesel engines, there has been increased activity in the development of diesel particulate filters, that is, exhaust emission filters for diesel engines. The role of a typical diesel particulate filter is to trap and remove the particulate components of the diesel exhaust stream, which include diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates, to prevent their discharge from the tailpipe.
There are a variety of diesel particulate filtration technologies on the market. For every diesel particulate filter, two performance aspects are crucial: the filtration efficiency of the system and the ability of the system to provide long-term operation without diminishing the filtration efficiency of the filter and performance of the engine. Factors that are critical to these performance aspects for a particular diesel particulate filter include whether the filter has the ability to handle temperatures up to 1400° C., a high capability to store soot and ash, a low pressure loss, a low thermal mass, stability, and durability. In addition, manufacturing costs and assembly volume are important considerations.
The filtration is achieved by a porous structure that allows transmission of the fluid phase but stops or captures diesel particulate matter larger than a threshold particle size. A variety of effective pore sizes are available and, accordingly, filters vary in their filtration efficiencies as a function of particle size of the diesel particulate matter.
Every filter has a finite capacity, and an overfilled diesel particulate filter can damage the engine through excessive exhaust backpressure and can itself be damaged or destroyed. Due to the low bulk density of diesel particulates, diesel particulate filters can quickly accumulate considerable volumes of soot. To prevent the filter pore clogging that causes backpressure to increase, thereby increasing load on the engine, the trapped particulate material is burned from the filter by continuous or periodic oxidation in the process of regeneration.
In most cases, thermal regeneration of diesel filters is employed, where the collected particulates are removed from the filter by oxidation to gaseous products. To ensure that particulates are oxidized at a sufficient rate, the filter must operate at a sufficient temperature and oxidizing gases, such as oxygen or nitrogen dioxide (or its precursors), must be supplied to the filter. In some filter systems, the source of heat (as well as of the oxidizing gases) is the exhaust gas stream itself. In this case, the filter, referred to as a passive filter, regenerates continuously during the regular operation of the engine. Passive filters usually incorporate some form of a catalyst, which lowers the soot oxidation temperature to a level that can be reached by exhaust gases during the operation of the vehicle. Another approach to facilitate reliable regeneration involves a number of active strategies of increasing the filter temperature (for example, engine management, electric heaters, microwaves, etc.). Regeneration of such systems, known as active filters, is usually performed periodically, as determined by the vehicle's control system.
Regeneration of filters that include a ceramic-based filter element, employing microwave energy, is also possible.
The most common type of diesel particulate filter structure is an extruded honeycomb monolithic structure, or wall-flow filter, in which the exhaust gases flow through walls between many small parallel channels, typically of square cross-section, so that the particulate matter accumulates on the upstream surface of the walls. Extruded wall-flow filters typically consist of a ceramic membrane. Because diesel exhaust is a high velocity gas stream, extruded wall-flow filters are not optimal structures because they exhibit high thermal mass, low porosity, and high backpressure.
Accordingly, there is a need to design filter assembly geometries for particulate filters for diesel exhaust treatment devices that can overcome one or more of the problems presented by current monolithic style filters, such as high thermal mass, high restriction, low capacity, poor mechanical and thermal durability, and limited packaging flexibility.